LOOKING FOR SIGNALS OF RECOVERY FROM THE END-PERMIAN MASS EXTINCTION THROUGH A TRANSGRESSIVE SEQUENCE FROM THE MIDDLE TRIASSIC ANISIAN OF NEVADA

The transition from ecosystems still undergoing recovery in the Early Triassic to fully recovered ecosystems in the Middle Triassic is not well constrained for eastern Panthalassa. While complex level-bottom and reef systems have been described elsewhere in the Middle Triassic there is evidence of lingering deleterious conditions and possible regional heterogeneity in the recovery of complex ecosystems recorded in the sedimentary units of the Western U.S.

We studied two sections of Anisian-aged mixed carbonate-siliciclastics from the Favret Formation in the Tobin Range and the Prida Formation in the Humboldt Range of western Nevada. Stratigraphic measurements, macrofacies, and petrographic analyses were used to develop a depositional model and investigate the complexity of level-bottom reef communities and evaluate the possibility of reef community development in this region.

Micrite was the primary component of the rock matrix and diagenetic replacement by microcrystalline quartz was common. The facies were predominately microbioclastic peloidal calcisiltites, peloidal grainstones, and bioclastic wackestones and packstones. Mottled textures at the macrofacies level reflect varying degrees of bioturbation throughout the section. Stratigraphy and petrography support a general shallowing trend through the section from a deep shelf setting at the base to intertidal deposits at the top of the section. The concentration of fossil content reflects this shallowing environmental gradient where deep shelf deposits are commonly composed of fecal pellets and abraded crinoid stem fragments. Inner shelf deposits contain more fossil fragments including bivalves, brachiopods, echinoderms, and gastropods.

We do not find evidence for the development of complex level-bottom or reef communities in either deep or shallow deposits of the Anisian in western Nevada. Our micrite-dominated environments are most representative of the initial stabilization phase before the colonization and diversification stages identified by Bonuso et al. (2018) for the late Middle Triassic and early Late Triassic of western Nevada. These results raise the question of what intrinsic or extrinsic conditions were delaying reef development.